What a great question this is, It all depends on where in the world you are asking this question from. In the USA a cookie is defined as a biscuit but in the UK a cookie is indeed defined as a cake, this is because a British biscuit by definition is dry and crunchy (twice baked or dehydrated), were as our cookies are moist, chewy, and certainly not dry, bringing them under the British definition of a cake

The reason why a cookie is a biscuit in the USA is due to there own defined terminology for products descriptions, in the same way the USA say chips and we say crisps or The USA say fries and we say chips, both nations refer to the same products but have different terminology’s for them. Therefore in the same way, our biscuit is Americas Cookie.

The British legal definition for a biscuit is as quoted: (curtesy of collinsdictionary.com)

” biscuit in British English(ˈbɪskɪt ) 1. British. a small flat dry sweet or plain cake of many varieties, baked from a dough. US and Canadian word: cookie. “

Also According to Wikipedia, A biscuit is a flour-based baked food product. In most countries, particularly in the Commonwealth and Ireland, biscuits are typically hard, flat and unleavened. They are usually sweet and may be made with sugar, chocolate, icing, jam, ginger or cinnamon. They can also be savoury and similar to crackers. Types of biscuit include sandwich biscuits, digestive biscuits, ginger biscuits, shortbread biscuits, chocolate chip cookies, chocolate-coated marshmallow treats, Anzac biscuits, biscotti and speculaas. In most of North America, nearly all hard sweet biscuits are called “cookies” (Curtesy of https://en.wikipedia.org/wiki/Biscuit)

Legally a cookie in England is not under the definition of a biscuit. Does this mean a cookie is a cake?

Cake by definition in England: An item of soft sweet food made from a mixture of flour, fat, eggs, sugar, and other ingredients, baked and sometimes iced or decorated.

The primary and legal difference between cake and biscuit is the moisture content, as our cookies are moist, chewy and soft they are classified as a cake.

Is a cookie a cake? YES in England they are a cake. After-all, who would put ice-cream with a biscuit.